Field of the Invention
The invention relates generally to molecular sieve based-catalysts used in selectively converting nitrogen oxides (NOx) present in a gas stream to nitrogen using a nitrogenous reductant such as ammonia (NH3) and in particular it relates to Fe-containing catalysts which are particularly active at relatively low temperatures in relation to conventional Fe zeolite catalysts. The molecular sieve in these catalysts is preferably a zeolite or a silicoaluminophosphate (SAPO).
Description of Related Art
Selective catalytic reduction (SCR) systems utilize NH3 as a reductant to reduce NOx to elemental nitrogen. A principal application of SCR technology is in the treatment of NOx emissions from internal combustion engines of motor vehicles, and especially lean-bum internal combustion engines. SCR systems are also applied to static sources of NOx, such as power plants.
One class of SCR catalysts is transition metal exchanged zeolites. Vanadium-based SCR catalysts are unsuited for higher temperature environments due to their thermal instability. This has led to the developments of copper and iron promoted zeolites. Copper zeolite catalysts achieve high NOx conversion (90% or more) at relatively low temperatures (from about 180 to about 250° C.), but they require the injection of greater amounts of urea to be effective at relatively higher temperatures (greater than about 450° C.). Conventional iron zeolite catalysts achieve high conversion (90% or more) of NOx at temperatures over 350° C., but at lower temperatures more typical for example of normal diesel engine exhaust (about 180 to about 250° C.), high conversions (up to about 90%) are obtained only in the presence of high levels of NO2 (50% of the total NOx levels, i.e. 1:1 NO2:NO).
It would therefore be desirable to provide SCR catalysts having improved low temperature (from about 200 to about 300° C.) performance.